The invention relates to the field of microelectronics and microsystems. It relates more specifically to a novel design of inductive components or integrated transformers which are intended to be associated with integrated circuits, such as those used especially in the radiofrequency field.
The purpose of the invention is more specifically to obtain inductors having a better Q-factor right from low frequencies and operating at higher frequencies than with inductors currently obtained.
As is known, integrated circuits are being used increasingly in the microwave and radiofrequency fields.
In these applications, it is important to be able to use tuned oscillating circuits consisting of a capacitor-inductor combination. However, these circuits must be produced so as to occupy increasingly smaller volumes. Furthermore, they must operate at increasingly higher frequencies.
Finally, the electrical consumption of such components is becoming a critical parameter, for example in cellular portable telephones, since this consumption has a direct influence on the autonomy of these appliances.
Thus, the passive components forming the filters used in radiofrequency systems, especially the inductors, are required to occupy as small as possible an area within the integrated circuits, with this inductor having as high an inductance as possible, and to result in as low as possible an electrical consumption.
Furthermore, it is known that the inductors fitted into integrated circuits made of semiconductor material are exposed to the influence of parasitic capacitances formed by the various localized substrate regions in the vicinity of the inductors.
FIG. 1 shows an equivalent circuit for an inductor implanted in an integrated-circuit chip.
Thus, such an inductor has an equivalent circuit in which, added to the actual inductor (L), there are the various parasitic components which make this inductor depart from its ideal behavior.
Thus, an inductor has a resistance (Rs) corresponding to that of the metal of which it is composed. Furthermore, this inductor has various capacitances (represented by the capacitors Ci and Co) which are parasitic capacitances resulting from the presence of the oxide layer deposited on top of the substrate. Furthermore, this inductor includes, in series with said capacitors Co and Ci, a capacitor Csub and a resistor Rsub in parallel, corresponding to the influence of the semiconductor substrate located between said oxide layer and the ground plane.
Moreover, a certain parasitic capacitance exists between the various turns making up the inductor and is modeled in the equivalent circuit in FIG. 1 by the capacitor (Cs).
The Applicant, in French Patent Application FR 98/08434, which at the date of filing of the present application has not yet been published, has described a solution allowing such inductors to be produced on a semiconductor substrate by adopting an arrangement allowing the capacitance of the interturn parasitic capacitor (Cs) to be greatly reduced. By virtue of such a solution it is possible to use such an inductor at higher frequencies, while still retaining satisfactory behavior.
It will be recalled that the optimum operating frequency is defined as being that for which the Q-factor is a maximum. The Q-factor is defined in a known manner by the ratio of the imaginary part or reactants to the real part of the input inductance corresponding to the model illustrated in FIG. 1.
The solution described in the aforementioned document, although satisfactory, does not allow the Q-factor to be significantly improved in the low-frequency ranges, that is to say those lying below half the optimum frequency, which is typically close to a few gigahertz in the radiofrequency applications of the invention.
This is because, in this frequency range, the behavior of the inductor is strongly dependent on the value (Rs) of the resistor, which corresponds to the electrical resistance of the metal strip from which the actual inductor is composed.
However, all the inductors produced in circuits, even integrated circuits, are made at the present time of aluminum, and are small in size having especially a very small thickness, thereby resulting in a high electrical resistance.
Thus, one of the problems that the invention aims to solve is that of the undesirable influence of the overall resistance of the winding forming the inductor.
The invention therefore relates to an inductive component, especially:
a substrate layer;
a flat inductor formed from a metal strip wound in a spiral.
The inductive component according to the invention is one wherein, on the one hand, the substrate layer is made of quartz and wherein, on the other hand, the metal strip is made of copper and has a thickness of greater than 10 microns.
In this way, the value of the resistance of the winding is very greatly reduced by using a conductive material much less resistive than the aluminum used in the prior art.
Furthermore, the chosen dimensional parameters, especially the thickness of the strip of which the turns are composed, also very greatly reduce the overall value of the resistance of the winding.
This reduction in the resistance is accomplished while retaining an extremely tiny parasitic capacitance (Cs) by virtue of the implantation of the metal strip on a substrate made of quartz, the dielectric properties of which are quite close to those of air.
By virtue of the characteristics of the inductor according to the invention, it has been observed that the Q-factor was ten times greater than that observed with an inductor of the same inductance but produced according to the prior art.
By way of example, for an inductance of about 5 nanohenries (nH) and for a typical frequency of 1.8 gigahertz (GHz), the Q-factor is about 40 whereas with the prior technologies it was close to 4.
In practice, it has been found that the results are very satisfactory when the thickness of the metal strip is about 30 microns.
According to one feature of the invention, the space between the opposing faces of two adjacent turns is free of material.
Consequently, the presence of air, which has a very low electrical permitivity, greatly limits the interturn parasitic capacitances which have been seen to have a negative impact on the optimum use of integrated inductors.
In one embodiment, the inductive component according to the invention furthermore includes a polyimide layer interposed between the inductor and the quartz substrate, within which polyimide layer that segment of the strip which connects the center of the spiral passes, and the end of the strip forming a connection terminal.
In practice, the metal strip is advantageously covered with a layer of gold on its faces other than those in contact with the substrate or the polyimide layer.
Thus, the risks of oxidation, inherent in the operation of the component according to the invention in a chemically aggressive environment, such as especially a wet, or indeed maritime, atmosphere, are overcome by protecting the conducting strip from the oxidation phenomena which would degrade the overall resistance of the strip.
The aforementioned aspects of the invention may also apply to the production of integrated transformers. Thus, such a transformer includes two flat inductors according to the invention, formed by two metal strips wound in spirals, said spirals being wound in each other so that the turns of one of the inductors are positioned between the turns of the other inductor.
In order to obtain high inductances, while still retaining a satisfactory Q-factor, in practice the strip is advantageously wound in two spirals in series, the two spirals being parallel to each other, the spiral closer to the substrate being embedded in a polyimide layer.
Consequently, by virtue of the magnetic coupling phenomena, the inductance is more than twice the inductance of an inductor formed from a single spiral. The overall size of such an inductor is therefore reduced. Furthermore, for a given inductance, two spirals are used, each having a resistance less than half of that of a single inductor, something which proves to be advantageous with regard to the Q-factor.
In one particular embodiment, intended for applications in particularly aggressive media, the polyimide layer in which the second spiral is embedded may be covered with a barrier layer made of silica. Consequently, the lifetime of such a component may be extended.
In one particular embodiment, the inductor according to the invention is such that the two ends of the strip form connection terminals on which spacer elements or xe2x80x9cbumpsxe2x80x9d having a height close to the thickness of the metal strip are mounted.
Consequently, such an inductor, or its transposition into the transformer, has elements making it easier for it to be fitted onto an integrated circuit.
The invention also relates to an integrated circuit associated with an aforementioned inductive component or integrated transformer.
According to one feature of the invention, such an integrated circuit comprises connection leads for the inductive component or for the integrated transformer and it is characterized in that:
the substrate used is made of quartz;
the metal strips are made of copper and have a thickness of greater than 10 microns.
In other words, the inductive component or the transformer are mounted using a technique known as xe2x80x9cflip-chipxe2x80x9d.
In practice, the spacer elements, usually called xe2x80x9cbumpsxe2x80x9d, are advantageously chosen so that they have a height close to the thickness of the strip of the inductive component.
Consequently, the strip forming the inductor is separated from the semiconductor substrate of the integrated circuit by a distance which ensures that the inductive component on the integrated circuit is mechanically stable, while limiting the influence of the parasitic conductivity of the semiconductor substrate on the behavior of the inductor.
This is because it has been observed that if the height of the spacer elements is too great, typically greater than the thickness of the strip, this then results in a risk of mechanical instability which may lead, in the case of shocks, to the connection between the integrated circuit and the inductor being broken.
Conversely, when the distance between the semiconductor substrate of the integrated circuit and the inductor is too small, phenomena of electrical loss through the semiconductor are observed, which reduce the performance of the latter.
Thus, in practice, good results are obtained with 30 micron spacers or xe2x80x9cbumpsxe2x80x9d for a strip thickness of about 30 microns.
In practice, in order to further improve the stability of the inductive component mounted on the integrated circuit, the spacer elements are advantageously in the form of a cylinder which preferably has a diameter close to three times its height.